Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the loss of upper and lower motor neurons, which lead to death between 1 to 6 years after becoming symptomatic. Mutations in superoxide dismutase (SOD) are linked to 2-3 percent of the cases of ALS and are the only proven cause of the disease. We found that these mutations weaken the structure of the protein, which decreases the affinity for zinc. Both wild type and mutant zinc-deficient SOD induce apoptosis in cultured motor neurons by a mechanism requiring nitric oxide. However, both wild type and mutant copper and zinc-containing SOD prevent motor neuron apoptosis induced by trophic factor deprivation. These results suggest that mutant SOD protein can be functional and protective when it is replete with copper and zinc but becomes toxic after the zinc atom is lost. We hypothesize that the gain-of-function that makes SOD toxic is due to the loss of zinc. We further hypothesize that wild type SOD might become zinc-deficient in sporadic ALS mediating motor neuron degeneration. Our results are controversial since they are in apparent contradiction with observation in transgenic mice carrying ALS-mutant SOD. We propose to test our hypothesis in motor neurons purified from transgenic mice over expressing wild type human SOD as well as mice carrying the G93A and G37R ALS-mutant SOD. We also will test the role of nitric oxide in the induction of motor neuron apoptosis by trophic factor deprivation and Zn-deficient SOD by using motor neurons purified from mice "knockout" for neuronal or endothelial nitric oxide synthase. The results of these investigations will help to solve some controversy on the mechanism of ALS-mutant SOD toxicity and will provide new insight in the mechanism of motor neuron degeneration in ALS that could result in the development of new therapeutic strategies.